Conventionally, for example, as shown in FIG. 1, Japanese Patent Application Laid-Open Publication No. 2009-266437 discloses a light source device which can include an LED module 103 configured to emit blue light as excitation light and a phosphor (such as a phosphor plate) 104 which is excited by the excitation light from the LED module 103 so as to emit yellow fluorescent light. The light source device is designed to mix the blue light and the yellow light to produce white light (pseudo white light). In this light source device, a method for improving the efficiency of light utilization has been suggested. The method can include disposing, between the LED module 103 and the phosphor plate 104, a wavelength selective filter 106 which transmits the blue light from the LED module 103 and reflects the yellow light emitted from the phosphor plate 104. Note that in FIG. 1, reference numeral 108 denotes a phosphor securing transparent body and reference numeral 107 denotes the beams of light (the excitation light and the fluorescent light) extracted from a light extraction surface 104a of the phosphor plate 104-.
In the structure of FIG. 1, the blue light from the LED module 103 can be incident upon the phosphor plate 104 after having been transmitted through the phosphor securing transparent body 108 and the wavelength selective filter 106. Part of the excitation light incident upon the phosphor plate 104 can be transmitted through the phosphor plate 104 and exit from the light extraction surface 104a of the phosphor plate 104. Meanwhile, the other part of the excitation light may be used in the phosphor plate 4 to excite the phosphor plate 4 to emit the above-described fluorescent light or may be reflected by the light extraction surface 104a and directed to the wavelength selective filter 106 so as to be transmitted through the wavelength selective filter 106. Accordingly, the blue light from the LED module 103 is emitted mainly from the portion of the phosphor plate 104 (excitation light irradiated spot) that is irradiated with the excitation light.
In contrast to this, the phosphor plate 104 isotropically emits the yellow light, so that part of the yellow light is emitted from the light extraction surface 104a of the phosphor plate 104, whereas the other part is reflected by the light extraction surface 104a and directed to the wavelength selective filter 106 so as to be selectively reflected by the wavelength selective filter 106. Then, part of the reflected light of the yellow light which has been selectively reflected by the wavelength selective filter 106 can exit from the light extraction surface 104a, whereas the other part of the light may be reflected by the extraction surface 104a and directed again to the wavelength selective filter 106 so as to become what is called multiple reflection light, which is further selectively reflected by the wavelength selective filter 106. Accordingly, the yellow light is emitted from the entire phosphor plate 104. In addition to this, the wavelength selective filter 106 causes the yellow light to be emitted at a greater intensity from the peripheral portion.
To facilitate assembly, the phosphor plate 104 is typically made larger than the excitation light irradiated spot. However, in this case, the yellow light emitting region having the entire phosphor plate 104 as an emission region is larger than the blue light emitting region which is generally the same in size as the excitation light irradiated spot. FIG. 2 is a view illustrating the phosphor plate 104 when it emits light. In FIG. 2, the beams of blue light and yellow light are mixed into white light in a region 110 which is generally the same in size as the excitation light irradiated spot. However, the yellow light is found in a region 111 outside the region 110 that is generally the same in size as the excitation light irradiated spot.
Accordingly, as shown in FIG. 3, with a lighting device which is designed such that the light source device of FIG. 1 is used for projection under magnification onto a predetermined projection surface using a lens assembly or the like, there was the problem in which variations in color of the light source are reflected by the projection surface, thus causing variations in radiation (variations in color) with the central portion being blue white and the peripheral portion being yellow. Note that in FIG. 3, reference numeral 102 denotes the lens assembly and reference numeral 105 denotes the projection surface.